In order to achieve good durability and related performance under environmental stress (cf. cyclic conditions of high temperature and humidity), present state-of-the-art, magnetic recording compositions tend to be cross-linked--especially where "premium" magnetic tape binder systems are involved. Cross-linking will be understood as the attachment of two chains of polymer molecules by bridges composed of either an element, a group or a compound which joins certain atoms of the chain by primary chemical bonds. It is recognized that cross-linking decreases elongation, increases strength, and enhances stability (cf. resistance to degradation from thermal, environmental, hydrolytic and/or chemical causes).
Currently, cross-linking is accomplished through a combination of chemical and thermal operations. The majority of current state-of-the-art binder systems utilize multifunctional isocyanates to produce an appropriate degree of cross-linking to various binders, or binder combinations, whereby to enhance resistance to environmental stresses consistent with the engendered recording application.
Typical prior art binder systems requiring cross-linking, generally using multifunctional isocyanates, to survive environmental stresses can be represented by the following classes of materials: polyurethanes, alone or in combination; phenoxy resins; vinyl resins such as vinyl chloride-vinyl acetate and vinyl chloride-vinyl acetate-vinyl alcohol, nitrocellulose, vinyl chloride-acrylonitrile, etc.
The value of cross-linking to achieve environmental stress stability has been described by R. J. Deffeyes in U.S. Pat. No. 4,020,227 as follows: "cross-linking also enhances the temperature stability of the coating. Consequently, it is desired that video recording tapes, tapes intended for use on high-speed computer transports and the like have cross-linked magnetic coating". A. Hosaka, Y. Isobe and K. Okuyama in U.S. Pat. No. 4,370,384 state that "conventional or thermoplastic combination of resins-urethane resin, vinyl chloride-vinyl acetate copolymer, cellulose resin, epoxy resin, phenoxy" lacks "satisfactory temperature and humidity resistance" (and) "reliability of storage of recorded data". Finally, M. Onizawa, in U.S. Pat. No. 4,128,539, in teachings not related to magnetic media, states that vinyl chloride resins require curing to increase thermal stability.
Here, we teach a magnetic coating that is not cross-linked, yet seems to have acceptable characteristics, including durability, even under environmental stress--something that will surprise workers. Workers will recognize that a thermoplastic system is preferable to a cross-linked one in many respects; e.g., more amenable to convenient fabrication methods (e.g., surface-finishing) and less variable over time (no pot life or other cure-continuance problems).